The X chromosome of Drosophila melanogaster is capable of at least five different levels of transcriptional activity: the male level which is twice that achieved in females (dosage compensation), an intermediate level in intersexes, a lower level in metafemales and a higher level in metamales. Since the X represents one fifth of the genome it constitutes a unique model system with which to study coordinate regulation of gene activity. Two major approaches are propsed: (1) an extensive study, by genetic and cytochemical means, to complete the characterization of the phenomenon, and to demonstrate the existence of the regulatory entities involved; (2) a multifaceted biochemical study of the molecular mechanisms responsible for compensation.